Physicians frequently use invasive medical devices, such as hand-held probes and catheters, in medical procedures to gain access to interior regions of the body. One type of procedure that has been becoming more widespread is tissue ablation. One example of a tissue ablation procedure is electrophysiology therapy, used to treat cardiac rhythm disturbances (arrhythmia).
In electrophysiology therapy, energy, such as radiofrequency (RF) energy, is used to ablate heart tissue to form a lesion that will block the propagation of aberrant electrical signals. To deliver the RF energy, energy transmitting elements, such as electrodes, are provided at the distal end of an ablation catheter. To access the cardiac tissue, a catheter, referred to as an introducer sheath, is percutaneously inserted into and advanced to the site of interest through a blood vessel, for example, such as the femoral artery. When the distal end of the introducer sheath is proximate the site of interest, the ablation catheter is inserted into the proximal end of the introducer sheath and advanced through the sheath. The distal end of the introducer sheath is open to allow the distal end of the ablation catheter, including the energy transmitting elements, to exit the sheath, and contact tissue at the site of interest. FIG. 1 is a view of a distal end of a typical ablation catheter 10 including an energy transmitting element 12 extending through a distal end 14 of an introducer sheath 16 and bearing against cardiac tissue 18 at an ablation site in a chamber of the heart 20. As shown, the distal end 14 of the introducer sheath 16 is typically straight. The energy transmitting element 12 may be one or more electrodes of tantalum, gold or platinum, for example, electrically connected to a radio frequency power source (not shown). Ablation catheters are described in U.S. Pat. Nos. 6,241,724 B1, 6,216,027 B1, and 6,004,269, for example, which are assigned to the assignee of the present invention and are incorporated by reference herein.
As shown in FIG. 1, only a portion of the circumferential surface of the electrode 12 is in contact with the tissue 20. The remainder of the electrode 12 is in contact with blood “B” circulating through the heart 20. Since the blood is conductive, much of the RF energy emitted by the electrode (as much as 75%) is thermally dissipated in the blood. The amount of RF energy that must be conveyed to electrodes in conventional ablation catheter systems to create even small therapeutic lesions is therefore quite high (up to 50 watts or more).
One or more temperature sensors 22 are typically provided on or in the electrode 12 to detect the temperature of the cardiac tissue being ablated. The temperature is used to control the application of energy to the electrode 12 during the procedure, typically under software control. The circulating blood in the heart acts as a heat sink that dissipates heat from the electrode 12 and from the tissue at the ablation site. Typically, there is a 15°-30° difference between the temperature measured by the sensor 22 and the actual temperature of the tissue. Thermal regulation of the ablation procedure is therefore difficult. Poor thermal regulation can lead to coagulum and popping of the cardiac tissue.
U.S. Pat. No. 6,241,724 B1, mentioned above, discloses an ablation catheter with multiple electrical segments arranged circumferentially around a distal end of the catheter. Each segment is electrically and thermally isolated from an adjacent segment. Independent signal wires are attached to each segment, enabling a physician to independently provide ablation energy to the one or more segments in contact with tissue. Dissipation of energy into the blood pool is decreased and energy efficiency and lesion creation are thereby improved.
Insulated wires have also been used as electrodes in ablation catheters. The insulation is removed from a portion of the circumference of the wire that will contact the tissue. The remainder of the wire, which may be exposed primarily to the circulating blood, is insulated. Less heat and RF energy is dissipated to the blood. Partially insulated coil electrodes are known, as well.
More energy efficient catheter ablation systems with simpler structures would be advantageous.